1. Field of the Invention
This invention relates generally to optical data storage systems and more particularly to maintaining the position of the light beam on the data tracks of such systems.
2. Description of the Prior Art
Optical data storage systems typically have an optical disk medium which stores data on concentric or spiral tracks. A fixed optical system (FOS) contains a laser for generating a light beam to be directed to the disk and sensors for detecting a reflected beam from the disk. The data is accessed by positioning a moveable optical head to a position proximate the desired track. The head is moved by a radial linear motor otherwise known as a course tracking actuator.
The optical head contains a rotatable mirror for directing the laser from the FOS to the disk and an objective lens for focussing the beam to the disk. The objective lens is attached to a focus actuator which moves the lens in order to keep the beam properly focussed on the disk. A fine tracking actuator in the optical head moves the rotatable mirror such that the beam of light always remains exactly on the desired track.
Examples of these types of optical systems include U.S. Pat. No. 4,466,088 issued Aug. 14, 1984 to Trethewey; U.S. Pat. No. 4,564,757 issued Jan. 14, 1986 to LaBudde, et al.; U.S. Pat. No. 4,744,071 issued May 10, 1988 to Bricot, et al.; U.S. Pat. No. 4,969,137 issued Nov. 6, 1990 to Sugiyama, et al.; and Japanese patent application JP 56-83850 published Jul. 8, 1981 by Nagashima, et al.
A problem with all of these systems is that both the focus actuator and the fine tracking actuator must be located on the optical head. This greatly increases the total mass of the optical head which in turn slows down the speed with which the course tracking actuator may position the optical head from track to track. The end result is that the optical storage systems have relatively slow access times compared to magnetic data storage systems.
Japanese patent application JP 02-216625 published Aug. 29, 1990 by Maedea, et al. proposes a way to remove the fine tracking actuator from the optical head. A rotatable mirror is positioned in the fixed optical system with a pivot point positioned a distance away from the reflecting point. By moving the mirror, the beam is displaced at the focus lens of the optical head and fine tracking is achieved. A problem with this scheme is that the aperture of the objective lens in the optical head is relatively small and the beam displacements caused by the rotatable mirror cause a large fraction of the beam to miss the aperture of the objective lens. This causes a large variation in the strength of the data signal which can result in misreading of the data.
What is needed is a fine tracking system which is not located on the optical head and which does not cause the beam to miss the aperture of the objective lens.